BUG #16978: Nested CTEs give ERROR in v13

The following bug has been logged on the website:

Bug reference: 16978
Logged by: Robins Tharakan
Email address: tharakan(at)gmail(dot)com
PostgreSQL version: 13.2
Operating system: Ubuntu
Description:

This narrowed down SQL (from what SQLSmith generated) seems to error out
only with N levels of CTE nesting - no tables / views / 1-row output.

The SQL works fine with v12.4 but raises an ERROR in v13 which seems like a
regression. Is this owing a recent change in v13 - that could affect this?

(This was found during a larger investigation around a v13 SegFault but this
does seem worthy of its own bug-report).

→ psql -h localhost
psql (14devel, server 13.2)
Type "help" for help.

postgres=# --Having w_err in the SQL gives an error about UNION ALL
postgres=# WITH RECURSIVE w6(c6) AS
postgres-# (WITH w6(c6) AS
postgres(# (WITH w8(c8) AS
postgres(# (WITH w9(c9) AS
postgres(# (WITH w10(c10) AS
postgres(# (WITH w11(c11) AS
postgres(# (WITH w_err(c12) AS
postgres(# (SELECT 1)
postgres(# SELECT * FROM w_err)
postgres(# SELECT * FROM w11)
postgres(# SELECT * FROM w10)
postgres(# SELECT * FROM w9)
postgres(# SELECT * FROM w8)
postgres(# SELECT * FROM w6)
postgres-# SELECT * FROM w6;
ERROR: recursive query "w6" does not have the form non-recursive-term UNION
[ALL] recursive-term
LINE 1: WITH RECURSIVE w6(c6) AS
^
postgres=#
postgres=#
postgres=#
postgres=# -- Removing w_err it works
postgres=# WITH RECURSIVE w6(c6) AS
postgres-# (WITH w6(c6) AS
postgres(# (WITH w8(c8) AS
postgres(# (WITH w9(c9) AS
postgres(# (WITH w10(c10) AS
postgres(# (WITH w11(c11) AS
postgres(# -- (WITH w_err(c12) AS
postgres(# (SELECT 1)
postgres(# -- SELECT * FROM w_err)
postgres(# SELECT * FROM w11)
postgres(# SELECT * FROM w10)
postgres(# SELECT * FROM w9)
postgres(# SELECT * FROM w8)
postgres(# SELECT * FROM w6)
postgres-# SELECT * FROM w6;
c6
----
1
(1 row)

postgres=> \q

→ r.sh 12
psql (14devel, server 12.4)
Type "help" for help.

postgres=> -- No error on v12 with or without w_err
postgres=>
postgres=> WITH RECURSIVE w6(c6) AS
postgres-> (WITH w6(c6) AS
postgres(> (WITH w8(c8) AS
postgres(> (WITH w9(c9) AS
postgres(> (WITH w10(c10) AS
postgres(> (WITH w11(c11) AS
postgres(> (WITH w_err(c12) AS
postgres(> (SELECT 1)
postgres(> SELECT * FROM w_err)
postgres(> SELECT * FROM w11)
postgres(> SELECT * FROM w10)
postgres(> SELECT * FROM w9)
postgres(> SELECT * FROM w8)
postgres(> SELECT * FROM w6)
postgres-> SELECT * FROM w6;
c6
----
1
(1 row)

postgres=> -- No error on v12 with or without w_err
postgres=>
postgres=> WITH RECURSIVE w6(c6) AS
postgres-> (WITH w6(c6) AS
postgres(> (WITH w8(c8) AS
postgres(> (WITH w9(c9) AS
postgres(> (WITH w10(c10) AS
postgres(> (WITH w11(c11) AS
postgres(> -- (WITH w_err(c12) AS
postgres(> (SELECT 1)
postgres(> -- SELECT * FROM w_err)
postgres(> SELECT * FROM w11)
postgres(> SELECT * FROM w10)
postgres(> SELECT * FROM w9)
postgres(> SELECT * FROM w8)
postgres(> SELECT * FROM w6)
postgres-> SELECT * FROM w6;
c6
----
1
(1 row)